1. Field of the Invention
This invention relates in general to the Fischer-Tropsch synthesis of hydrocarbons from synthesis gas and more particularly to a process of catalytically converting olefins and light hydrocarbons in Fischer-Tropsch tail gas to more desirable products.
2. Description of Related Art
The Fischer-Tropsch synthesis for the production of hydrocarbons is now well known and described in the technical and patent literature. In this synthesis, a gas composed primarily of CO and H2 (commonly referred to as synthesis gas or syngas) is reacted under catalytic conditions to yield a wide variety of gaseous, liquid and solid hydrocarbonaceous products. Significant amounts of light hydrocarbons (C3 and C4) are produced along with the more valuable C5+ hydrocarbons. These light hydrocarbons are recovered, purified and sold as Liquefied Petroleum Gas (LPG). However, LPG is of lower value than the C5+ products. Also, there are significant costs associated with liquefying and transporting LPG.
To provide a stable operation, the Fischer-Tropsch synthesis normally operates under reaction conditions which only partially convert the synthesis gas. Part of the unreacted synthesis gas (commonly known as tail gas) is recycled to the Fischer-Tropsch reactor. Another part of the unreacted synthesis gas is recycled to the synthesis gas generator where CO2 contained in the unreacted synthesis gas is used to achieve the desired H2/CO stoichiometry in the synthesis gas. Still another part of the unreacted synthesis gas is used as fuel gas or simply flared.
The tail gas contains LPG and other light hydrocarbons mixed with unconverted synthesis gas and byproducts such as CO2 and water. Recovery of LPG and light hydrocarbons fractions from this stream is difficult because they are relatively dilute (less than about 10 wt. %). An example of the complexity of recovering light hydrocarbon fractions from a Fischer-Tropsch effluent is described in U.S. Pat. No. 4,049,741. Recovery often requires the use of expensive cryogenic techniques. The LPG and light hydrocarbons from the Fischer-Tropsch process are fairly olefinic, i.e., greater than about 50% each of C3 and C4 olefins with typical olefin concentrations of the C3 fraction being 75% and the C4 fraction being 70%. When the tail gas containing olefinic LPG and light hydrocarbons is recycled to the Fischer-Tropsch reactor, a modest amount of these materials is incorporated into the growing hydrocarbon chain but the majority of the light olefins are saturated to form paraffins which consumes valuable hydrogen. When the olefinic LPG and light olefins are recycled to the synthesis gas reactor, they can contribute to coking. Olefins and C3+ hydrocarbons in particular are associated with coke formation.
In addition to the aforementioned problems associated with the recovery of LPG and light hydrocarbons from tail gas, there are additional problems associated with the Fischer-Tropsch synthesis. Traces of catalytically poisonous nitrogen compounds are formed in the syngas generation step. These catalytically poisonous nitrogen compounds include nitrites, most commonly HCN. Ammonia is another catalytically poisonous nitrogen compound present in synthesis gas. Molecular nitrogen is also present but it is not a catalyst poison. The formation of catalytically poisonous nitrogen compounds during syngas generation is described in U.S. Pat. Nos. 6,063,349 and 5,929,126. The nitrogen compounds deactivate the Fischer-Tropsch catalysts which then require that more catalyst and larger reactor volumes be used than would otherwise be necessary. In addition to deactivating the catalyst, the nitrogen compounds are incorporated into the products of the reaction, thus making it more difficult to upgrade the products into more salable products. While molecular nitrogen is itself not a poison, Fischer Tropsch synthesis certain conditions, small portions of the molecular nitrogen can be converted to ammonia, which is a catalyst poison. Thus, removal of catalytically poisonous nitrogen compounds from both gas feeds to the Fischer-Tropsch and gas product streams (which are recycled) must be considered.
From an operational and economic standpoint, a more economical technique to recover at least a portion of the LPG and olefinic light hydrocarbons from the unreacted synthesis gas from the Fischer-Tropsch unit and convert them into more valuable products would be highly desirable. Improvements in the conventional recovery techniques using cryogenic separation would be highly desirable. An economical technique to remove at least a portion of catalytically poisonous nitrogen compounds from synthesis gas prior to entry of the syngas into the Fischer-Tropsch unit likewise would be a desirable feature.
It is an object of the invention to provide an improved process for recovering LPG and light hydrocarbons from Fischer-Tropsch tail gas.
It is another object of the invention to develop a procedure to nitrogen-containing catalyst poisons from synthesis gas prior to entry into Fischer-Tropsch reactors.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, the claims appended thereto, and the figures of the drawing.
Light olefins including propylene and butenes in the tail gas from a Fischer-Tropsch process are catalytically condensed (oligomerized and/or aromatized) to form higher molecular weight products including naphtha-boiling range olefins, aromatics and combinations thereof. The condensation is an oligomerization/ aromatization reaction using an acidic catalyst. Due to their higher molecular weight, the products of the condensation are easier to recover from the tail gas. The products of the condensation are separated into a C5+ product stream and unreacted syngas mixed with CO2. At least part of the unreacted syngas is recycled to the Fischer-Tropsch reactor and/or the syngas generator. In this way, the LPG fraction is upgraded to provide more valuable C5+ products.
In one embodiment of the invention, the oligomerization/aromatization condensation can be conducted on the original synthesis gas feed to the Fischer-Tropsch reactor. In this manner, a significant portion of the catalytically poisonous nitrogen compounds are removed. The poisonous compounds may accumulate on the catalyst that converts the olefins but the catalyst can be regenerated easily by stripping with hot gases or by burning in air.
By virtue of the techniques of the invention, LPG and other light hydrocarbons are more readily separated from the tail gas fraction of a Fischer-Tropsch synthesis while being simultaneously converted to more valuable C5+ products. Additionally, the present invention offers the advantage of removing at least a portion of catalytically poisonous nitrogen compounds present in a syngas feed to a Fischer-Tropsch reactor.